Preparation of methane sulfonic acid

ABSTRACT

A process for preparing methanesulfonic acid by irradiating a mixture comprising acetic acid, sulfur dioxide and oxygen with light, wherein the reaction mixture is irradiated with an average cumulative irradiance in the range from 240 to 320 mm of from 0.05 to 50 mmol quanta/cm 2 h at the light entry area.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an improved process for preparingmethanesulfonic acid from a mixture comprising acetic acid, sulfurdioxide and oxygen by irradiation with light.

Methanesulfonic acid is the simplest representative of the class ofalkanesulfonic acids and is of great use for a large number ofindustrial applications such as the production of metal coatings byelectrodeposition or else as esterification catalyst.

2. Description of the Related Art

The most widely used processes for preparing methanesulfonic acid arethe oxidation of methyl mercaptan or dimethyl disulfide with oxygen orwith chlorine to give methanesulfonyl chloride, followed by hydrolysis.All these processes are associated with problems of toxicity and odor,because the starting materials are formed from hydrogen sulfide, whichproblems can be overcome only with great technical complexity.

DE 907 053 describes the irradiation of carboxylic acids in the presenceof air and sulfur dioxide. The reaction products are the correspondingβ-sulfo carboxylic acids.

By contrast, irradiation of acetic acid at room temperature in thepresence of air and sulfur dioxide takes a different course, asdescribed in Tetrahedron Lett. 1966, 3095. In the reaction,methanesulfonic acid and also 60%, based on the methanesulfonic acidformed, of sulfuric acid are obtained. However, an industrial processwith formation of such a large amount of by-product is uneconomic.

BRIEF SUMMARY OF THE INVENTION

It is an object of the present invention to provide an economic processwhich affords methanesulfonic acid in good yield and in which not morethan 50%, based on the methanesulfonic acid, of sulfuric acid is formed.

We have found that this object is achieved by a process for preparingmethanesulfonic acid by irradiating a mixture comprising acetic acid,sulfur dioxide and oxygen with light, wherein the reaction mixture isirradiated with an average cumulative irradiance in the range from 240to 320 nm of from 0.05 to 50 mmol quanta/cm²h at the light entry area.

DETAILED DESCRIPTION OF THE INVENTION

The lamps preferably employed are those emitting light in the range from240 to 320 nm, such as low pressure mercury lamps, preferably high andmedium pressure mercury lamps, either pure or doped, which arecommercially available and whose radiated power is from 125 watts to 60kW. Also suitable are excimer lamps, which are preferably used in thewavelength range from 240 to 320 nm in which sulfur dioxide shows strongabsorption.

Further suitable light sources are halogen lamps, gas discharge lamps orfluorescent tubes.

The average cumulative irradiance in the range from 240 to 320 nm at thearea where the light enters the reaction mixture is not more than 50mmol quanta/cm²h, preferably not more than 10 mmol quanta/cm²h, and theeffectiveness is optimal with irradiances of up to 5 mmol quanta/cm²h.Irradiances below 0.05 mmol quanta/cm²h slow down the reaction. In apreferred embodiment, irradiation is carried out with an irradiance of0.1 mmol quanta/cm²h or above.

It is easily possible with knowledge of the quanta flux of the lamp,which is usually stated by the manufacturer, to select a reactor with asuitable irradiation area depending on the lamp output and the requiredamount to be converted.

Thus, a conventional 150 W high pressure mercury lamp has in thewavelength range of 240-320 nm a quanta flux of 0.128 mol quanta/h and a700 W lamp has one of 0.6 mol quanta/h.

It is generally known to ensure thorough mixing in irradiationreactions, in particular at the zone of entry of the light into thereaction mixture.

Thorough mixing is achieved, for example, by producing turbulent flownear the wall or high velocities near the wall in the liquid. This canbe achieved by reactor tubes which are curved in the region ofirradiation, for example in the form of a loop or coil, when thereaction gases are bubbled through the liquid. Effective mixing cangenerally be achieved by passing the reaction gases as a stream of finegas bubbles through the reaction mixture. This effect can furthermore beachieved or additionally assisted if the liquid in turn is transportedthrough the reactor. It is also advantageous to use a gas back-mixingstirrer (hollow shaft stirrer).

Preferred types of reactor for the process are, for example, tubularreactors, tube bundle reactors, loop reactors or cocurrent packedcolumns, which are generally known to the skilled worker.

In a particularly preferred embodiment, the reaction mixture iscirculated, for example by pumps or by stirrers, as is the case in theloop reactor. The reaction mixture advantageously flows through thereactor with flow rates of from 0.01 m/s to 1 m/s.

In principle, the various types of loop reactors decribed in “ChemischeReaktionstechnik, Lehrbuch der technischen Chemie”, Volume 1, ThiemeVerlag, Stuttgart, New York 1992, pages 257-262, are suitable. Theliquid can moreover be conveyed in an inner or outer circulation.

The process according to the invention can be carried out eithercontinuously or batchwise.

It is is advantageous if the thickness of the layer of liquid to beirradiated is a multiple of the depth of penetration of the relevantradiation. The thickness of the layer of liquid to be irradiated ispreferably at least 1 cm, particularly preferably at least 5 cm. It islikewise economically worthwhile, in order to reduce the holdup, if thethickness of the layer of liquid to be irradiated is chosen to be nolarger than 150 cm, preferably no larger than 50 cm.

In industrial equipment, the light source is arranged in front ofappropriately transparent windows, such as quartz glass, in the reactionvessels or, preferably as lamps immersed centrally or radially in thereaction chambers.

It is moreover perfectly possible to use more than one lamp if a largeramount of substance is to be reacted. It is also possible in principleto arrange a plurality of lamps in one reactor (Ullmann's encyclopediaof industrial Chemistry, Vol A 19, 5th edition, VCH, Weinheim, 1991,pages 573 to 586.)

In an advantageous embodiment, a loop reactor is chosen, in which casethe reaction mixture is conveyed past a plurality of lamps insuccession. For reasons of space, the lamps are advantageously arrangedparallel to one another to result in a coil reactor. It is preferred inthis case to use immersion lamps, but it is likewise possible by asuitable arrangement to convey the reaction mixture between theindividual lamps. If a coiled reactor tube is used, the lamp ispreferably located in the winding axis of the coil.

It is particularly advantageous to use a loop reactor with an immersionlamp arranged concentrically inside, or a tube bundle reactor where theindividual tubes are arranged in the form of a circle around a lightsource, and the reaction mixture flows through each of themalternatively in parallel or in series. In the latter case, it isadvantageous to design the individual tubes in the tube bundle to beflattened toward the light source in order to minimize radiation lossesthrough diffraction or reflection.

To improve utilization of the radiation emitted by the radiation source,or else to promote or inhibit individual steps in the reactions takingplace in the reaction mixture, and thus finally to increase the yield,it is possible to add auxiliaries to the reaction mixture.

In a preferred embodiment, sensitizers or photoinitiators are added tothe mixture as auxiliaries which permit long wavelength radiation to beused exclusively or additionally. Examples of conventionalphotoinitiators are:

thermal initiator systems such as hydrogen peroxide, peroxides orazoisobutyronitrile,

ketones such as acetophenone, benzophenone or benzanthrone,

acyloins such as benzoin derivatives,

α-diketones such as diacetyl, phenanthrenequinone or benzil,

quinones such as anthraquinone derivatives, sulfur compounds such asdiphenyl disulfide or

tetramethylthiuram disulfide,

halogen compounds such as chlorine, bromotrichloromethane, bromoform orstyrene dibromide,

metal carbonyls and perchloro compounds such as manganese carbonyl andorganic halogen compounds,

and hexaarylbisimidazole, fatty acid iodine salts, α-diketone monooximeesters, triphenylphosphine, organic sulfinic acids and dyes,bis(arylsulfonyl)diazomethane, Bunte salts or uranyl salts.

Further preferred auxiliaries are nitrogen oxides, chloroform and thehydroxides and the inorganic or organic salts of copper or cobalt, suchas copper or cobalt hydroxide, copper or cobalt sulfate, copper orcobalt sulfite, copper or cobalt carbonate, copper or cobaltbicarbonate, copper or cobalt methanesulfonate or, preferably, copper orcobalt acetate.

The auxiliaries are, as a rule, employed in a concentration of from 10⁻²to 10% by weight, preferbly 10⁻¹ to 5% by weight, in each case based onacetic acid.

Acetic acid in purities available industrially is suitable for thereaction.

The reaction of acetic acid with sulfur dioxide and oxygen in thepresence of light is carried out by ensuring that the mixing of thegases with the acetic acid is as homogeneous as possible. Thus, finedispersion of the reaction gases can be brought about by known measures,eg. by introduction through nozzles.

The ratio of sulfur dioxide to oxygen may moreover vary. As a rule,equimolar amounts of the gases are passed through the reaction mixture.However, an up to 10 molar excess of one of the gases is also possible.In general, it is advantageous to use acetic acid which is approximatelysaturated with gas in the temperature range according to the invention.

In a preferred embodiment, 0.01 to 10 mol of sulfur dioxide per mole ofacetic acid are passed in continuously during the irradiation period.The same applies to the oxygen. The two gases can be introduced asmixture or separately.

Sulfur dioxide can moreover be fed into the reaction in pure form,liquid or gaseous, or as solution in acetic acid or a suitable solvent,such as water.

It is possible to add sulfur dioxide in the gas mixture with oxygen oran oxygen-containing gas mixture, such as air, to the reaction. The useof a sulfur dioxide/air mixture results in lower partial pressures ofthe sulfur dioxide and of the oxygen, but this has no detectable effecton the process according to the invention because, as a rule, aceticacid, sulfur dioxide and oxygen are present in excess relative to theeffective amount of radiation.

Sulfur dioxide and oxygen or sulfur dioxide and air are preferablypassed in together.

To disperse the gases or the gas mixture in the liquid, it is possibleto use conventional distributors such as sintered disks, perforatedplates, sieve trays and nozzles, preferably glass frits or annularnozzles.

Gas separation takes place in a conventional way after the gas haspassed through the irradiation zone.

The process according to the invention is substantially independent ofpressure. Elevated pressure can be used, for example up to 20 bar,preferably up to 15 bar. It is likewise possible to choose a slightreduction in pressure down to, for example, 500 mbar. A reduction inpressure diminishes the concentration of sulfur dioxide dissolved in theacetic acid, so that an SO₂ concentration which is too low reduces theconversion. The pressure range from 1 to 10 bar is particularlypreferred.

The process according to the invention can advantageously be carried outabove 0° C., preferably 20° C., in particular 50° C. Side reactionspredominate above 160° C., so that the irradiation is preferably carriedout at up to 140° C., particularly preferably up to 120° C. Thetemperature of the mixture can be for example, as generally known,controlled using a jacket on the light source side and a UV-transparentmedium such as water.

The reaction can be carried out in inert diluents, where “inert”includes the fact that the diluent has negligible intrinsic absorptionin the wavelength range used.

However, the reaction is preferably carried out without diluent.

The irradiation time depends on the amount of acetic acid used, thetemperature, pressure and the radiated power of the lamps. For example,1 l of reaction solution is irradiated with a 150 watt high pressuremercury lamp with a light emission of 0.128 mol quanta/h cumulativelyfrom 240 to 320 mm for from 15 minutes to 20 hours, preferably from 1 to10 hours.

It may, because of by-product formation, be advantageous to continue thereaction only as far as partial conversion. The reaction is preferablycarried out until the methanesulfonic acid content in the totaldischarge is 20% by weight, in particular 15% by weight.

The product is isolated in a conventional way, as a rule bydistillation. It is preferred for acetic acid to be distilled out in afirst distillation stage and for the methanesulfonic acid to bedistilled out from the bottom product in a second stage. Thedistillations can in each case be carried out batchwise or continuously,preferably both continuously. The distillations can be carried out underpressures from 0.01 mbar to 1 bar, preferably between 0.1 and 100 mbar.The acetic acid removed in this way if reaction is incomplete, possiblymixed with traces of methanesulfonic acid and/or sulfuric acid, can bereused in this form. Furthermore, the workup is preferably designed sothat the sulfuric acid byproduct can be utilized in sulfuric acidcleavage plants.

The process according to the invention can be carried out, for example,by irradiating the acetic acid in a coiled quartz glass tube reactorwith, located in the winding axis of the coiled tube, a high pressuremercury lamp. The start and end of the coiled tube are connected by anoutside line so that the reaction mixture is circulated by a pump. Thetwo gases are continuously passed in, flowing in the same direction asthe liquid.

In a preferred embodiment, a tubular reactor with a high pressuremercury immersion lamp is chosen in place of the coiled tube reactor,with an otherwise identical arrangement. The following examples areintended to illustrate the process according to the invention in detail.

The process according to the invention provides a good yield and a highcontent of methanesulfonic acid product and thus good selectivity.

EXAMPLE 1

Apparatus

A coiled quartz glass tube with an internal diameter of 1 cm and a tubelength of 1.6 m and and internal diameter of the coil of 7 cm has itsends connected by an outside line with incorporated pump. A 150 watthigh pressure mercury lamp is located in the winding axis of the coiledtube. 300 g of acetic acid were circulated in this reactor at anascending flow rate of 80 l/h. At the inlet to the coiled tube, 10standard-condition liters of air/h and 10 standard-condition liters ofsulfur dioxide/h were continuously passed in together and removeddownstream of the outlet through a gas/liquid separator. The reactionmixture was irradiated at 90° C. for 6.5 h and then worked up bydistillation. Analysis was by quantitative ion chromatography.

Table 1 below summarizes the reaction conditions and results of thetests in Example 1 and in Examples 2 to 4 which were carried outsimilarly.

TABLE 1 Reaction conditions and results of the photochemical preparationof methanesulfonic acid in a coiled tube reactor Acetic Addition to re-H₂SO₄/ Rate T acid action MSA MSA ratio [MSA Ex. [° C.] [g] [% byweight] [g] by weight g/kWh] 1 90 300 — 20.8 35/100 15 2 90 330 0.1%13.9 23/100 14 Cu(OCOCH₃)₂ 3 90 314 5% CHCl₃ 20.6 31/100 21 MSA =Methanesulfonic acid.

A test carried out as in Example 1 at 60° C. with 330 g of acetic acidas starting compound afforded 18.4 g of methanesulfonic acid.Furthermore, irradiation as in Example 1 at 30° C. afforded 14.0 g ofmethanesulfonic acid.

EXAMPLE 4 (COMPARATIVE)

100 g of acetic acid in a cylindrical flask (height 14 cm, diameter 4.5cm) made of quartz glass, besides which was located a 150 Watt highpressure mercury lamp with a quartz cooling jacket at a distance of 5cm, were irradiated while stirring with a magnetic stirrer at 90° C. for6.5 h. The complete system was surrounded by a reflecting foil in orderto prevent losses of light. During the irradiation, a mixture of sulfurdioxide and air (10 l(STP)/h SO₂, 10 l(STP)/h air) was passed incontinuously. Working up took place as described above.

The discharge comprised 6 g with a content of 31% by weight MSA{circumflex over (=)} 1.86 g MSA {circumflex over (=)} 1.9 g/kWh MSA.

EXAMPLE 5

In a vertically arranged tubular pressure reactor (length 18 cm,internal diameter 8 cm) with pressure-resistant quartz glass tube(external diameter 4 cm, wall thickness 5 mm) which is concentricallyarranged therein and which is connected to both the upper and lower lidspressure-tight in such a way that a light source can be introducedthrough an appropriate hole in the lid into the interior of the tube,acetic acid and, a concentrically arranged annular nozzle, a gas mixtureof sulfur dioxide (40 l (STP)/h) and air (100 l (STP/h) are passedco-currently upward under pressure. The mixture leaving the reactor isdecompressed, and the liquid components are recycled. 1100 g of aceticacid were circulated in this reactor, being pumped upward at a flow rateof 80 to 100 l/h. The reaction mixture was irradiated with a 150 Whigh-pressure mercury vapor lamp at 90° C. under 4 bar for 6.5 h. 39 gof methanesulfonic acid (40 g/kWh) were obtained, with a ratio ofsulfuric acid to methanesulfonic acid of 50/100 by weight.

EXAMPLES 6 to 8

In a vertically arranged tubular reactor (length 25 cm, internaldiameter 15 cm) with quartz glass tube (external diameter 6 cm) which isconcentrically arranged therein and which is connected to the upper lidin such a way that a high-pressure mercury vapor lamp can be introducedthrough an appropriate opening in the lid into the interior of the tube,acetic acid and, through a glass annular nozzle or frit arranged in themiddle of the base of the reactor, sulfur dioxide and a nitrogen/oxygenmixture are passed co-currently upward. The liquid components of thedischarge from the reactor are recycled. 4200 g of acetic acid werecirculated in this reactor, being pumped upward at a flow rate of 80 l/hat 90° C. The reaction mixture was irradiated with high-pressure mercuryvapor lamps at 90° C. under atmospheric pressure for 6.5 h.

Quanta fluxes of the lamps (240 to 320 nm)=The 150 W lamp has a quantaflux of 0.128 mol quanta/h. The 700 W lamp has a quanta flux of 0.6 molquanta/h.

TABLE 2 Reaction conditions and experimental results for Examples 6 to 9SO₂ H₂SO₄/MSA MSA Gas intro- [L(STP)/ MSA ratio by rates Ex. duction h]N₂/O₂ Lamp [g] weight [g/kWh] 6 Annular 40 8/2, 150 W  48 39/100 71nozzle 401(STP)/h 7 Glass frit 20 8/2, 700 W 199 8.5/100  44 201(STP)/h8 Glass frit 40 8/2, 700 W 399 23/100 88 1601(STP)/h

What is claimed is:
 1. A process for preparing methanesulfonic acid byirradiating a mixture comprising acetic acid, sulfur dioxide and oxygen,wherein the cumulative irradiance in the range from 240 to 320 nmaverages from 0.1 to 50 mmol quanta/cm²h at the area where the lightenters the reaction mixture.
 2. A process as claimed in claim 1, whereinthe reaction is carried out in a loop reactor.
 3. A process as claimedin claim 1, wherein the radiation source is a lamp immersed in thereaction mixture.
 4. A process as claimed in claim 1, whereinauxiliaries are added to the reaction mixture.
 5. A process as claimedin claim 1, wherein the reaction is carried out under an absolutepressure of up to 20 bar.
 6. A process as claimed in claim 1 where thecumulative irradiance where the light enters the reaction mixture is notmore than 10 mmol quanta/cm²h.
 7. A process as claimed in claim 1wherein the reaction is carried out at from 0 to 160° C.
 8. A process asclaimed in claim 7, wherein the radiation source is a lamp immersed inthe reaction mixture.
 9. A process as claimed in claim 7, whereinauxiliaries are added to the reaction mixture.
 10. A process as claimedin claim 7, wherein the reaction is carried out under an absolutepressure of up to 20 bar.